The present invention concerns an intervertebral disc prosthesis designed to be substituted for fibrocartilaginous discs ensuring connection between the vertebra of the vertebral column or the end of the latter.
The intervertebral discs are formed from a deformable but noncompressible element called “nucleus pulposus” containing approximately 80% water, surrounded by several elastic fibrous layers converging to maintain the nucleus, absorb part of the forces applied to the entire disc and stabilize the articulation. These elements may often be broken down or damaged by compression, displacement or wear and tear, following shocks, infections, exaggerated forces or simply over time.
The breakdown of this articulation element may cause intense pain and significant constraint in the patient. Beyond the surgery that consisted of blocking the deficient articulation and possibly purely and simply removing the damaged disc, a therapeutic route for the last twenty or so years consists of surgically replacing the defective disc with a functional prosthesis. However, the use of such a prosthesis requires a device that is not very cumbersome, that supports significant forces, or has a great sturdiness over time. Furthermore, the comfort of the patients already affected by great and acute pain makes it desirable to arrange for a prosthesis that most faithfully reproduces the natural possibilities of movements and at the same time ensures the best stability possibility to the spinal column that is sometimes already damaged.
The use of such a prosthesis therefore crucially depends on the stability that it allows the spinal column, as much during movements as during static forces or lengthy constant position.
A certain number of prostheses have been proposed with a compressible material base, with the goal of reproducing the kinematics of natural movement while reproducing its components and their characteristics of shape or plasticity, as described in the patent FR 2 124 815 which proposes a disc from elastomer material reinforced by a textile material. These devices present the drawback of a lifetime that is often too limited and also suffer from drawbacks indeed due to this elasticity. In fact, since the prosthesis is entirely compressible, a progressive sliding of the prosthesis may be produced relative to the vertebra between which it is placed, which too often leads it to leave its housing. The addition of anchoring pins does not allow sufficient remedy for this problem, because the micromovements permitted by the compressibility of the material of the prosthesis also include a vertical component, which too easily allows the pins to leave their housing with each movement.
Among the prostheses nor resting on the deformation of materials, a type of prosthesis frequently used is described by the patent DE 30 23 353 and is formed of a nucleus with the shape of a biconvex lens forming articulation between two plates each presenting a cavity with a shape approximately complementary to the nucleus in their centre and on their perimeter a shoulder retaining this nucleus. This arrangement presents the advantage by comparison to a more limited ball-and-socket joint of using significant contact surface, which largely decreases the wear and tear.
To incline one with the other on one side, the plates are articulated by their internal cavity on the edge of the nucleus of the side in question, but according to a rotation movement which, on the other side makes their edges move apart more than they were at rest. This separating has a tendency to detach the vertebral plates on which they are supported, which damages the surface of the vertebra at the sites where the plates have just anchored and again allows progressive displacement with risk of complete ejection of the prosthesis.
Another type of prosthesis described in patent FR 2 659 226 consists of an upper plate presenting a concave face that comes to slide on a nucleus in the form of a segment of a sphere, this nucleus being immobilized in a cavity of the lower plate. In this case, the rotation is done more satisfactorily from the point of view of space of the plates, but the sliding of the upper plate on a sphere whose centre is located on the exterior of the prosthesis also causes lateral displacement which may be harmful as much to the kinematics of movement as to the organs present in the vicinity.
A solution is proposed in the patent FR 2 730 159 in the form of a nucleus presenting two spherical faces, oriented in the same direction, and with different radius. The nucleus with cylindrical exterior slides on a convex surface belonging the lower plate and itself presents a convex surface on the top, on which the upper plate slides. Because the nucleus is movable horizontally, it is in a position to move apart from one side when the plates approach the other. However, this device presents the drawback of risking the complete ejection of the nucleus outside the prosthesis, this drawback also existing in the device described by the patent DE 30 23 353.
In the goal of limiting the risks of ejection of the nucleus, the patents WO 00 53 127, as well as U.S. Pat. No. 5,401,269 and U.S. Pat. No. 4,759,766 propose to provide a translation stop, produced in different ways.
In certain variants, a translation stop is disclosed in the form of a relief protruding from one contact surface of the nucleus and movable in a recess in the plate or inversely. This type of internal stop is therefore located on the interior of a contact or support surface between nucleus and plate, and therefore decreases the available surface considerably. This decrease in support surface increases the stresses undergone by the materials, therefore the risks of wear and tear or strain in creep or exceeding elastic limits. The separation between the support surface and housing receiving the stop may also risk marking the piece which is supported above and damaging the latter.
In certain cases, such a central stop is provided with a noncircular shape, which in a certain extent allows the rotations of the nucleus to be limited relative to the plate that provides it. However, this noncircular forms additional constraint which again limits the surface available for support. Furthermore, the angles of this shape themselves form fragile zones, which only ensures low sturdiness to this stop operation in rotation.
In other variants, a collar protrudes from the nucleus and surrounds it in the space between the two plates. In its exterior part this collar widens at a certain height along the axis of the spinal column towards each of the plates, which forms two interior borders that may be supported on the exterior border of contact surfaces of these same plates. However, this type of external peripheral stop presents certain drawbacks, in particular in terms of obstruction.
In fact, the configuration of this collar represents considerable vertical obstruction (along the spinal column axis) and the contact surfaces of the plates must also present a certain height to be able to stop this collar in translation. Furthermore, the peripheral shape of this type of stop also occupies considerable radial space, in particular in a section plane where the spinal column presents the smallest width, as in sagittal plane. Given the limited space available in the disc, or intervertebral, space, this obstruction may occupy a space that would be useful for the configuration of the rest of the prosthesis, which may limit the results in terms of kinematics or reliability.
Moreover, this type of external peripheral stop requires a nucleus with biconvex shape to be used, to allow for provision of sufficient height for the contact surfaces of the plates to form an exterior border usable by this stop. Therefore, this type of stop is difficult to produce for a nucleus presenting one or more concave surfaces, while such forms of nucleus may allow the kinematics of the prosthesis to be made more comfortable with use by the patient.
In the case where the contact surfaces between nucleus and plates are not circular, such a collar may also be able to limit the clearance in rotation of the nucleus relative to the plates, for example by peripheral contact between two concentric ellipses and with different radii. However, such contact is done according to a very tight angle between the surfaces being supported on each other, which makes the position of this limit not very precise and increases the risks of wear and tear or blockage by clamping. Furthermore, the clearance in rotation permitted by such kinematics is directly dependent on the clearance permitted in translation, and may not be chosen independently of the latter during design of the prosthesis.